For many cardiological investigations, the spatially-resolved imaging of the blood supply to the myocardium by means of medical imaging techniques is an important tool for diagnostic back-up. A typical application example is the continuing checking of the effects of stenoses in the coronary arteries which, for example, have been identified previously in a cardiological computed tomography examination. In order to be able to estimate correctly the prognosis for the patient, it is helpful to investigate stenoses of this type with regard to their hemodynamic relevance. An answer needs to be found to the question of whether the blood supply through a stenosis is reduced at rest and/or under exertion and whether the myocardium is undersupplied or whether a still sufficient blood supply exists.
Normally, nowadays, such perfusion measurements to investigate the blood supply to the myocardium are carried out using magnetic resonance tomography methods (“MR perfusion investigations”) or techniques of nuclear medicine, for example, SPECT. However, MR perfusion investigations are only available in specialist centers and are also very complex and costly. The imaging in nuclear medicine techniques such as SPECT suffers from low spatial resolution. They also often result in ambiguous or falsely positive findings. Alternatively, an assessment of the hemodynamic relevance of stenoses can be made through evaluation of the “Fractional Flow Reserve”. Here, in the context of angiographic investigations by catheter, the pressure behind and in front of a stenosis are measured and, from these values, the pressure ratio (known as the Fractional Flow Reserve) is calculated. Investigations of this type have the disadvantage that they are invasive.
Computed tomography methods also essentially permit assessments to be made of the blood supply to the myocardium. For this purpose, the heart can be investigated with a conventional cardiac computed tomography protocol, for example, following injection of a, for example, iodine-containing contrast medium. A computed tomography protocol within the meaning of embodiments of the present invention should be understood to be a collection of control commands according to which the computed tomography system (hereinafter called “CT system”) is controlled automatically during measurement, once started. Such protocols or measurement protocols are known to persons skilled in the art. In order to investigate the local blood supply to the myocardium, the CT values of the image pixels in the CT images of the myocardium can be evaluated, for example, by visual inspection or using suitable software methods.
In a healthy myocardium, it can be taken that the contrast medium is evenly distributed through the myocardium and therefore the CT values of all the image pixels of the myocardium are evenly elevated. Regions in the myocardium in which the image pixels have lower CT values than the surrounding myocardium can be interpreted as regions with reduced contrast medium uptake and therefore as zones of reduced blood supply, i.e. as having perfusion defects. Unfortunately, however, a local relative lowering in the CT values in the myocardium can also have other causes than a reduced contrast medium uptake, for example, a locally elevated fat content in the myocardium. Therefore with this investigation technique, a relative reduction in CT values for other reasons, cannot be distinguished from a genuine perfusion defect.
One possibility for solving this problem would be to perform the cardiac CT contrast medium recordings using a “dual-energy method”. Here, the recordings of the tissue region of interest, that is, during a myocardial examination of the heart, are investigated with two different X-ray spectra, so that a CT raw data set is recorded with at least one first X-ray energy or with a first X-ray spectrum and a second CT raw data set is recorded with a second X-ray energy. A multi-energy method can also be used, in which yet further image data sets are recorded at still other energies. From the different CT raw data sets, a plurality of different image data sets which reproduce the CT values of the imaged tissue regions and of the contrast medium at the different X-ray spectra is reconstructed. Whereas fat and soft tissues have very similar CT values at different X-ray spectra, the CT value of most contrast media, particularly iodine, significantly increases with decreasing X-ray energy. Due to this strong change in the CT value with different X-ray spectra, it is possible to determine the contrast medium content per image pixel quantitatively and to reproduce this in a “virtual contrast medium” image. In a virtual contrast medium image data set, therefore, the contrast medium content of each individual image pixel in the myocardium can be given quantitatively as a measure for the local blood supply. In this way, regions in the myocardium in which less contrast medium has been taken up due to a perfusion defect, can be readily identified with high sensitivity. However, evaluation of the images following such a method is relatively severely laden with artifacts. Due to often unavoidably high contrast medium concentrations, for example, in the right ventricle of the heart, artifacts arise in a contrast medium image data set of this type which can mimic a locally reduced iodine concentration in the myocardium and can therefore simulate a perfusion defect. Due to other inconsistencies between the CT images recorded with different X-ray spectra, for example, beam-hardening, pixel artifacts can arise during calculation of the contrast medium content which, as sites of darkening in the myocardium, mimic perfusion defects in the myocardium.